1. Field of the Invention
The invention relates to a video signal discriminating circuit for discriminating whether a video signal which is inputted is in a standard screen or a wide screen format and a television receiver which has such a video discriminating circuit.
2. Description of the Related Art
In a television receiver of a wide screen having an aspect ratio of 16:9, besides a video source (hereinafter, referred to as a standard screen) of the screen of the existing NTSC system having an aspect ratio of 4:3, video signals of various aspect ratios such as video source of the screen having an aspect ratio of 16:9, video source (hereinafter, referred to as a wide screen) of a cinema size having an aspect ratio of 2:1, and the like are inputted. The television receiver has a function to switch a display mode in accordance with the aspect ratios of those video sources. In case of switching the display mode in accordance with such video sources, if it is switched by a manual operation, the operation is complicated and it is difficult to use. Therefore, a method of discriminating whether the inputted video signal is in the standard screen or the wide screen and switching the display mode in accordance with a discrimination result is used.
The discrimination about whether the inputted video signal is in the standard screen or wide screen can be performed by checking, for example, whether there are black stripe portions at the upper and lower positions of the screen or not. That is, FIGS. 1A and 1B show a standard screen and a wide screen. As shown in FIG. 1A, although there is no black stripe at the upper and lower positions of the screen in case of the ordinary screen, as shown in FIG. 1B, in case of the wide screen, black stripes B1 and B2 are caused at the upper and lower positions of the screen. Therefore, whether the screen is in the standard screen or the wide screen format can be discriminated by detecting video signal levels of the upper and lower positions of the screen and checking whether the upper and lower video signal levels of the screen are at the black level or not.
As mentioned above, in case of discriminating whether the screen is in the standard screen or the wide screen format by detecting whether there are the black stripe portions at the upper and lower positions of the screen, there is a case where it is difficult to discriminate in dependence on the scene. For example, in a scene of a night view, the whole screen is set to almost the black level. In such a case, it is difficult to discriminate whether the screen is in the standard screen format or the wide screen by checking whether the black stripe portions exist at the upper and lower positions of the screen. Even in a case where the upper and lower portions of the screen have a certain degree of luminance instead of the black stripes or a case where a caption signal of a a speech or the like has been superimposed in the upper and lower black stripes of the screen, it is difficult to discriminate about the standard screen or the wide screen.
Therefore, to prevent an erroneous discrimination between the standard screen and the wide screen in accordance with the scene, a method of obtaining a histogram of the video signal level of each line and discriminating about the standard screen or the wide screen has been proposed.
As mentioned above, whether the inputted video signal is of the standard screen or the wide screen can be discriminated by checking whether there are the black stripe portions at the upper and lower positions of the screen or not. However, when a signal to noise (S/N) ratio of the input video signal is low or the like, it is difficult to discriminate by checking whether there are the black stripe portions at the upper and lower positions of the screen or not.
That is, in a case where the video source is a reproduction signal of a VTR such that the dubbing has been repeated or a case where a television broadcasting is received in a weak electric field, the S/N ratio deteriorates and noises are generated on the screen. Therefore, if the S/N ratio of the video signal deteriorates, noises are mixed in the black stripe portions, the black stripe portion does not become perfect black, and a point of a high luminance is mixed. It is, therefore, difficult to discriminate whether the screen is the standard screen or the wide screen.
The method of discriminating about the standard screen or the wide screen by obtaining the histogram of the video signal level of each line is effective for discrimination in the scene of a night view because the feature of the picture plane is detected and whether the screen is the standard screen or the wide screen can be discriminated. In a case where the luminance level of the black stripe portions is high, since the discrimination luminance obtained by the histogram is also high, such a method is also effective in a case where the black stripe portion is not at the perfect black level. However, since the amplitude of noises is generally widely distributed in both of the upper and lower portions for the original signal, the discrimination luminance obtained from the histogram cannot take into consideration of an influence by the noises.